Changes

Video for Linux Two API Specification: Revision 0.24
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# Chapter 6. Changes

The following chapters document the evolution of the V4L2 API, errata or extensions. They are also intended to help application and driver writers to port or update their code.

# 6.1. Differences between V4L and V4L2

The Video For Linux API was first introduced in Linux 2.1 to unify and replace various TV and radio device related interfaces, developed independently by driver writers in prior years. Starting with Linux 2.5 the much improved V4L2 API replaces the V4L API, although existing drivers will continue to support V4L applications in the future, either directly or through the V4L2 compatibility layer in the `videodev` kernel module translating ioctls on the fly. For a transition period not all drivers will support the V4L2 API.

## 6.1.1. Opening and Closing Devices

For compatibility reasons the character device file names recommended for V4L2 video capture, overlay, radio, teletext and raw vbi capture devices did not change from those used by V4L. They are listed in Chapter 4 and below in Table 6-1.

The V4L `videodev` module automatically assigns minor numbers to drivers in load order, depending on the registered device type. We recommend that V4L2 drivers by default register devices with the same numbers, but the system administrator can assign arbitrary minor numbers using driver module options. The major device number remains 81.

Table 6-1. V4L Device Types, Names and Numbers

Device Type | File Name | Minor Numbers
--- | --- | ---
Video capture and overlay | `/dev/video` and `/dev/bttv0`a, `/dev/video0` to `/dev/video63` | 0-63
Radio receiver | `/dev/radio`b, `/dev/radio0` to `/dev/radio63` | 64-127
Teletext decoder | `/dev/vtx`, `/dev/vtx0` to `/dev/vtx31` | 192-223
Raw VBI capture | `/dev/vbi`, `/dev/vbi0` to `/dev/vbi31` | 224-255
Notes: a. According to Documentation/devices.txt these should be symbolic links to `/dev/video0`. Note the original bttv interface is not compatible with V4L or V4L2. b. According to `Documentation/devices.txt` a symbolic link to `/dev/radio0`.

V4L prohibits (or used to prohibit) multiple opens of a device file. V4L2 drivers may support multiple opens, see Section 1.1 for details and consequences.

V4L drivers respond to V4L2 ioctls with an EINVAL error code. The compatibility layer in the V4L2 `videodev` module can translate V4L ioctl requests to their V4L2 counterpart, however a V4L2 driver usually needs more preparation to become fully V4L compatible. This is covered in more detail in Chapter 5.

## 6.1.2. Querying Capabilities

The V4L `VIDIOCGCAP` ioctl is equivalent to V4L2's `VIDIOC_QUERYCAP`.

The `name` field in struct `video_capability` became `card` in struct v4l2_capability, `type` was replaced by `capabilities`. Note V4L2 does not distinguish between device types like this, better think of basic video input, video output and radio devices supporting a set of related functions like video capturing, video overlay and VBI capturing. See Section 1.1 for an introduction.

struct `video_capability` `type` | struct v4l2_capability `capabilities` flags | Purpose
--- | --- | ---
`VID_TYPE_CAPTURE` | `V4L2_CAP_VIDEO_CAPTURE` | The video capture interface is supported.
`VID_TYPE_TUNER` | `V4L2_CAP_TUNER` | The device has a tuner or modulator.
`VID_TYPE_TELETEXT` | `V4L2_CAP_VBI_CAPTURE` | The raw VBI capture interface is supported.
`VID_TYPE_OVERLAY` | `V4L2_CAP_VIDEO_OVERLAY` | The video overlay interface is supported.
`VID_TYPE_CHROMAKEY` | `V4L2_FBUF_CAP_CHROMAKEY` in field `capability` of struct v4l2_framebuffer | Whether chromakey overlay is supported. For more information on overlay see Section 4.2.
`VID_TYPE_CLIPPING` | `V4L2_FBUF_CAP_LIST_CLIPPING` and `V4L2_FBUF_CAP_BITMAP_CLIPPING` in field `capability` of struct v4l2_framebuffer | Whether clipping the overlaid image is supported, see Section 4.2.
`VID_TYPE_FRAMERAM` | `V4L2_FBUF_CAP_EXTERNOVERLAY` not set in field `capability` of struct v4l2_framebuffer | Whether overlay overwrites frame buffer memory, see Section 4.2.
`VID_TYPE_SCALES` | `-` | This flag indicates if the hardware can scale images. The V4L2 API implies the scale factor by setting the cropping dimensions and image size with the `VIDIOC_S_CROP` and `VIDIOC_S_FMT` ioctl, respectively. The driver returns the closest sizes possible. For more information on cropping and scaling see Section 1.11.
`VID_TYPE_MONOCHROME` | `-` | Applications can enumerate the supported image formats with the `VIDIOC_ENUM_FMT` ioctl to determine if the device supports grey scale capturing only. For more information on image formats see Chapter 2.
`VID_TYPE_SUBCAPTURE` | `-` | Applications can call the `VIDIOC_G_CROP` ioctl to determine if the device supports capturing a subsection of the full picture ("cropping" in V4L2). If not, the ioctl returns the EINVAL error code. For more information on cropping and scaling see Section 1.11.
`VID_TYPE_MPEG_DECODER` | `-` | Applications can enumerate the supported image formats with the `VIDIOC_ENUM_FMT` ioctl to determine if the device supports MPEG streams.
`VID_TYPE_MPEG_ENCODER` | `-` | See above.
`VID_TYPE_MJPEG_DECODER` | `-` | See above.
`VID_TYPE_MJPEG_ENCODER` | `-` | See above.

The `audios` field was replaced by `capabilities` flag `V4L2_CAP_AUDIO`, indicating if the device has any audio inputs or outputs. To determine their number applications can enumerate audio inputs with the `VIDIOC_G_AUDIO` ioctl. The audio ioctls are described in Section 1.5.

The `maxwidth`, `maxheight`, `minwidth` and `minheight` fields were removed. Calling the `VIDIOC_S_FMT` or `VIDIOC_TRY_FMT` ioctl with the desired dimensions returns the closest size possible, taking into account the current video standard, cropping and scaling limitations.

## 6.1.3. Video Sources

V4L provides the `VIDIOCGCHAN` and `VIDIOCSCHAN` ioctl using struct `video_channel` to enumerate the video inputs of a V4L device. The equivalent V4L2 ioctls are `VIDIOC_ENUMINPUT`, `VIDIOC_G_INPUT` and `VIDIOC_S_INPUT` using struct v4l2_input as discussed in Section 1.4.

The `channel` field counting inputs was renamed to `index`, the video input types were renamed as follows:

struct `video_channel` `type` | struct v4l2_input `type`
--- | ---
`VIDEO_TYPE_TV` | `V4L2_INPUT_TYPE_TUNER`
`VIDEO_TYPE_CAMERA` | `V4L2_INPUT_TYPE_CAMERA`

Unlike the `tuners` field expressing the number of tuners of this input, V4L2 assumes each video input is connected to at most one tuner. However a tuner can have more than one input, i. e. RF connectors, and a device can have multiple tuners. The index number of the tuner associated with the input, if any, is stored in field `tuner` of struct v4l2_input. Enumeration of tuners is discussed in Section 1.6.

The redundant `VIDEO_VC_TUNER` flag was dropped. Video inputs associated with a tuner are of type `V4L2_INPUT_TYPE_TUNER`. The `VIDEO_VC_AUDIO` flag was replaced by the `audioset` field. V4L2 considers devices with up to 32 audio inputs. Each set bit in the `audioset` field represents one audio input this video input combines with. For information about audio inputs and how to switch between them see Section 1.5.

The `norm` field describing the supported video standards was replaced by `std`. The V4L specification mentions a flag `VIDEO_VC_NORM` indicating whether the standard can be changed. This flag was a later addition together with the `norm` field and has been removed in the meantime. V4L2 has a similar, albeit more comprehensive approach to video standards, see Section 1.7 for more information.

## 6.1.4. Tuning

The V4L `VIDIOCGTUNER` and `VIDIOCSTUNER` ioctl and struct `video_tuner` can be used to enumerate the tuners of a V4L TV or radio device. The equivalent V4L2 ioctls are `VIDIOC_G_TUNER` and `VIDIOC_S_TUNER` using struct v4l2_tuner. Tuners are covered in Section 1.6.

The `tuner` field counting tuners was renamed to `index`. The fields `name`, `rangelow` and `rangehigh` remained unchanged.

The `VIDEO_TUNER_PAL`, `VIDEO_TUNER_NTSC` and `VIDEO_TUNER_SECAM` flags indicating the supported video standards were dropped. This information is now contained in the associated struct v4l2_input. No replacement exists for the `VIDEO_TUNER_NORM` flag indicating whether the video standard can be switched. The `mode` field to select a different video standard was replaced by a whole new set of ioctls and structures described in Section 1.7. Due to its ubiquity it should be mentioned the BTTV driver supports several standards in addition to the regular `VIDEO_MODE_PAL` (0), `VIDEO_MODE_NTSC`, `VIDEO_MODE_SECAM` and `VIDEO_MODE_AUTO` (3). Namely N/PAL Argentina, M/PAL, N/PAL, and NTSC Japan with numbers 3-6 (sic).

The `VIDEO_TUNER_STEREO_ON` flag indicating stereo reception became `V4L2_TUNER_SUB_STEREO` in field `rxsubchans`. This field also permits the detection of monaural and bilingual audio, see the definition of struct v4l2_tuner for details. Presently no replacement exists for the `VIDEO_TUNER_RDS_ON` and `VIDEO_TUNER_MBS_ON` flags.

The `VIDEO_TUNER_LOW` flag was renamed to `V4L2_TUNER_CAP_LOW` in the struct v4l2_tuner `capability` field.

The `VIDIOCGFREQ` and `VIDIOCSFREQ` ioctl to change the tuner frequency where renamed to `VIDIOC_G_FREQUENCY` and `VIDIOC_S_FREQUENCY`. They take a pointer to a struct v4l2_frequency instead of an unsigned long integer.

## 6.1.5. Image Properties

V4L2 has no equivalent of the `VIDIOCGPICT` and `VIDIOCSPICT` ioctl and struct `video_picture`. The following fields where replaced by V4L2 controls accessible with the `VIDIOC_QUERYCTRL`, `VIDIOC_G_CTRL` and `VIDIOC_S_CTRL` ioctls:

struct `video_picture` | V4L2 Control ID
--- | ---
`brightness` | `V4L2_CID_BRIGHTNESS`
`hue` | `V4L2_CID_HUE`
`colour` | `V4L2_CID_SATURATION`
`contrast` | `V4L2_CID_CONTRAST`
`whiteness` | `V4L2_CID_WHITENESS`

The V4L picture controls are assumed to range from 0 to 65535 with no particular reset value. The V4L2 API permits arbitrary limits and defaults which can be queried with the `VIDIOC_QUERYCTRL` ioctl. For general information about controls see Section 1.8.

The `depth` (average number of bits per pixel) of a video image is implied by the selected image format. V4L2 does not explicitely provide such information assuming applications recognizing the format are aware of the image depth and others need not know. The `palette` field moved into the struct v4l2_pix_format:

struct `video_picture` `palette` | struct v4l2_pix_format `pixfmt`
--- | ---
`VIDEO_PALETTE_GREY` | `V4L2_PIX_FMT_GREY`
`VIDEO_PALETTE_HI240` | `V4L2_PIX_FMT_HI240`a
`VIDEO_PALETTE_RGB565` | `V4L2_PIX_FMT_RGB565`
`VIDEO_PALETTE_RGB555` | `V4L2_PIX_FMT_RGB555`
`VIDEO_PALETTE_RGB24` | `V4L2_PIX_FMT_BGR24`
`VIDEO_PALETTE_RGB32` | `V4L2_PIX_FMT_BGR32`b
`VIDEO_PALETTE_YUV422` | `V4L2_PIX_FMT_YUYV`
`VIDEO_PALETTE_YUYV`c | `V4L2_PIX_FMT_YUYV`
`VIDEO_PALETTE_UYVY` | `V4L2_PIX_FMT_UYVY`
`VIDEO_PALETTE_YUV420` | None
`VIDEO_PALETTE_YUV411` | `V4L2_PIX_FMT_Y41P`d
`VIDEO_PALETTE_RAW` | Nonee
`VIDEO_PALETTE_YUV422P` | `V4L2_PIX_FMT_YUV422P`
`VIDEO_PALETTE_YUV411P` | `V4L2_PIX_FMT_YUV411P`f
`VIDEO_PALETTE_YUV420P` | `V4L2_PIX_FMT_YVU420`
`VIDEO_PALETTE_YUV410P` | `V4L2_PIX_FMT_YVU410`
Notes: a. This is a custom format used by the BTTV driver, not one of the V4L2 standard formats. b. Presumably all V4L RGB formats are little-endian, although some drivers might interpret them according to machine endianess. V4L2 defines little-endian, big-endian and red/blue swapped variants. For details see Section 2.4. c. `VIDEO_PALETTE_YUV422` and `VIDEO_PALETTE_YUYV` are the same formats. Some V4L drivers respond to one, some to the other. d. Not to be confused with `V4L2_PIX_FMT_YUV411P`, which is a planar format. e. V4L explains this as: "RAW capture (BT848)" f. Not to be confused with `V4L2_PIX_FMT_Y41P`, which is a packed format.

V4L2 image formats are defined in Chapter 2. The image format can be selected with the `VIDIOC_S_FMT` ioctl.

## 6.1.6. Audio

The `VIDIOCGAUDIO` and `VIDIOCSAUDIO` ioctl and struct `video_audio` are used to enumerate the audio inputs of a V4L device. The equivalent V4L2 ioctls are `VIDIOC_G_AUDIO` and `VIDIOC_S_AUDIO` using struct v4l2_audio as discussed in Section 1.5.

The `audio` "channel number" field counting audio inputs was renamed to `index`.

On `VIDIOCSAUDIO` the `mode` field selects one of the `VIDEO_SOUND_MONO`, `VIDEO_SOUND_STEREO`, `VIDEO_SOUND_LANG1` or `VIDEO_SOUND_LANG2` audio demodulation modes. When the current audio standard is BTSC `VIDEO_SOUND_LANG2` refers to SAP and `VIDEO_SOUND_LANG1` is meaningless. Also undocumented in the V4L specification, there is no way to query the selected mode. On `VIDIOCGAUDIO` the driver returns the actually received audio programmes in this field. In the V4L2 API this information is stored in the struct v4l2_tuner `rxsubchans` and `audmode` fields, respectively. See Section 1.6 for more information on tuners. Related to audio modes struct v4l2_audio also reports if this is a mono or stereo input, regardless if the source is a tuner.

The following fields where replaced by V4L2 controls accessible with the `VIDIOC_QUERYCTRL`, `VIDIOC_G_CTRL` and `VIDIOC_S_CTRL` ioctls:

struct `video_audio` | V4L2 Control ID
--- | ---
`volume` | `V4L2_CID_AUDIO_VOLUME`
`bass` | `V4L2_CID_AUDIO_BASS`
`treble` | `V4L2_CID_AUDIO_TREBLE`
`balance` | `V4L2_CID_AUDIO_BALANCE`

To determine which of these controls are supported by a driver V4L provides the `flags` `VIDEO_AUDIO_VOLUME`, `VIDEO_AUDIO_BASS`, `VIDEO_AUDIO_TREBLE` and `VIDEO_AUDIO_BALANCE`. In the V4L2 API the `VIDIOC_QUERYCTRL` ioctl reports if the respective control is supported. Accordingly the `VIDEO_AUDIO_MUTABLE` and `VIDEO_AUDIO_MUTE` flags where replaced by the boolean `V4L2_CID_AUDIO_MUTE` control.

All V4L2 controls have a `step` attribute replacing the struct `video_audio` `step` field. The V4L audio controls are assumed to range from 0 to 65535 with no particular reset value. The V4L2 API permits arbitrary limits and defaults which can be queried with the `VIDIOC_QUERYCTRL` ioctl. For general information about controls see Section 1.8.

## 6.1.7. Frame Buffer Overlay

The V4L2 ioctls equivalent to `VIDIOCGFBUF` and `VIDIOCSFBUF` are `VIDIOC_G_FBUF` and `VIDIOC_S_FBUF`. The `base` field of struct `video_buffer` remained unchanged, except V4L2 defines a flag to indicate non-destructive overlays instead of a `NULL` pointer. All other fields moved into the struct v4l2_pix_format `fmt` substructure of struct v4l2_framebuffer. The `depth` field was replaced by `pixelformat`. See Section 2.4 for a list of RGB formats and their respective color depths.

Instead of the special ioctls `VIDIOCGWIN` and `VIDIOCSWIN` V4L2 uses the general-purpose data format negotiation ioctls `VIDIOC_G_FMT` and `VIDIOC_S_FMT`. They take a pointer to a struct v4l2_format as argument. Here the `win` member of the `fmt` union is used, a struct v4l2_window.

The `x`, `y`, `width` and `height` fields of struct `video_window` moved into struct v4l2_rect substructure `w` of struct `v4l2_window`. The `chromakey`, `clips`, and `clipcount` fields remained unchanged. Struct `video_clip` was renamed to struct v4l2_clip, also containing a struct `v4l2_rect`, but the semantics are still the same.

The `VIDEO_WINDOW_INTERLACE` flag was dropped. Instead applications must set the `field` field to `V4L2_FIELD_ANY` or `V4L2_FIELD_INTERLACED`. The `VIDEO_WINDOW_CHROMAKEY` flag moved into struct v4l2_framebuffer, under the new name `V4L2_FBUF_FLAG_CHROMAKEY`.

In V4L, storing a bitmap pointer in `clips` and setting `clipcount` to `VIDEO_CLIP_BITMAP` (-1) requests bitmap clipping, using a fixed size bitmap of 1024 × 625 bits. Struct `v4l2_window` has a separate `bitmap` pointer field for this purpose and the bitmap size is determined by `w.width` and `w.height`.

The `VIDIOCCAPTURE` ioctl to enable or disable overlay was renamed to `VIDIOC_OVERLAY`.

## 6.1.8. Cropping

To capture only a subsection of the full picture V4L defines the `VIDIOCGCAPTURE` and `VIDIOCSCAPTURE` ioctls using struct `video_capture`. The equivalent V4L2 ioctls are `VIDIOC_G_CROP` and `VIDIOC_S_CROP` using struct v4l2_crop, and the related `VIDIOC_CROPCAP` ioctl. This is a rather complex matter, see Section 1.11 for details.

The `x`, `y`, `width` and `height` fields moved into struct v4l2_rect substructure `c` of struct `v4l2_crop`. The `decimation` field was dropped. In the V4L2 API the scaling factor is implied by the size of the cropping rectangle and the size of the captured or overlaid image.

The `VIDEO_CAPTURE_ODD` and `VIDEO_CAPTURE_EVEN` flags to capture only the odd or even field, respectively, were replaced by `V4L2_FIELD_TOP` and `V4L2_FIELD_BOTTOM` in the field named `field` of struct v4l2_pix_format and struct v4l2_window. These structures are used to select a capture or overlay format with the `VIDIOC_S_FMT` ioctl.

## 6.1.9. Reading Images, Memory Mapping

### 6.1.9.1. Capturing using the read method

There is no essential difference between reading images from a V4L or V4L2 device using the `read()` function, however V4L2 drivers are not required to support this I/O method. Applications can determine if the function is available with the `VIDIOC_QUERYCAP` ioctl. All V4L2 devices exchanging data with applications must support the `select()` and `poll()` functions.

To select an image format and size, V4L provides the `VIDIOCSPICT` and `VIDIOCSWIN` ioctls. V4L2 uses the general-purpose data format negotiation ioctls `VIDIOC_G_FMT` and `VIDIOC_S_FMT`. They take a pointer to a struct v4l2_format as argument, here the struct v4l2_pix_format named `pix` of its `fmt` union is used.

For more information about the V4L2 read interface see Section 3.1.

### 6.1.9.2. Capturing using memory mapping

Applications can read from V4L devices by mapping buffers in device memory, or more often just buffers allocated in DMA-able system memory, into their address space. This avoids the data copying overhead of the read method. V4L2 supports memory mapping as well, with a few differences.

V4L | V4L2
--- | ---
| The image format must be selected before buffers are allocated, with the `VIDIOC_S_FMT` ioctl. When no format is selected the driver may use the last, possibly by another application requested format.
Applications cannot change the number of buffers. The it is built into the driver, unless it has a module option to change the number when the driver module is loaded. | The `VIDIOC_REQBUFS` ioctl allocates the desired number of buffers, this is a required step in the initialization sequence.
Drivers map all buffers as one contiguous range of memory. The `VIDIOCGMBUF` ioctl is available to query the number of buffers, the offset of each buffer from the start of the virtual file, and the overall amount of memory used, which can be used as arguments for the `mmap()` function. | Buffers are individually mapped. The offset and size of each buffer can be determined with the `VIDIOC_QUERYBUF` ioctl.
The `VIDIOCMCAPTURE` ioctl prepares a buffer for capturing. It also determines the image format for this buffer. The ioctl returns immediately, eventually with an EAGAIN error code if no video signal had been detected. When the driver supports more than one buffer applications can call the ioctl multiple times and thus have multiple outstanding capture requests. The `VIDIOCSYNC` ioctl suspends execution until a particular buffer has been filled. | Drivers maintain an incoming and outgoing queue. `VIDIOC_QBUF` enqueues any empty buffer into the incoming queue. Filled buffers are dequeued from the outgoing queue with the `VIDIOC_DQBUF` ioctl. To wait until filled buffers become available this function, `select()` or `poll()` can be used. The `VIDIOC_STREAMON` ioctl must be called once after enqueuing one or more buffers to start capturing. Its counterpart `VIDIOC_STREAMOFF` stops capturing and dequeues all buffers from both queues. Applications can query the signal status, if known, with the `VIDIOC_ENUMINPUT` ioctl.

For a more in-depth discussion of memory mapping and examples, see Section 3.2.

## 6.1.10. Reading Raw VBI Data

Originally the V4L API did not specify a raw VBI capture interface, only the device file `/dev/vbi` was reserved for this purpose. The only driver supporting this interface was the BTTV driver, de-facto defining the V4L VBI interface. Reading from the device yields a raw VBI image with the following parameters:

struct v4l2_vbi_format | V4L, BTTV driver
--- | ---
sampling_rate | 28636363 Hz NTSC (or any other 525-line standard); 35468950 Hz PAL and SECAM (625-line standards)
offset | ?
samples_per_line | 2048
sample_format | V4L2_PIX_FMT_GREY. The last four bytes (a machine endianess integer) contain a frame counter.
start[] | 10, 273 NTSC; 22, 335 PAL and SECAM
count[] | 16, 16a
flags | 0
Notes: a. Old driver versions used different values, eventually the custom `BTTV_VBISIZE` ioctl was added to query the correct values.

Undocumented in the V4L specification, in Linux 2.3 the `VIDIOCGVBIFMT` and `VIDIOCSVBIFMT` ioctls using struct `vbi_format` were added to determine the VBI image parameters. These ioctls are only partially compatible with the V4L2 VBI interface specified in Section 4.7.

An `offset` field does not exist, `sample_format` is supposed to be `VIDEO_PALETTE_RAW`, equivalent to `V4L2_PIX_FMT_GREY`. The remaining fields are probably equivalent to struct v4l2_vbi_format.

Apparently only the Zoran (ZR 36120) driver implements these ioctls. The semantics differ from those specified for V4L2 in two ways. The parameters are reset on `open()` and `VIDIOCSVBIFMT` always returns an EINVAL error code if the parameters are invalid.

## 6.1.11. Miscellaneous

V4L2 has no equivalent of the `VIDIOCGUNIT` ioctl. Applications can find the VBI device associated with a video capture device (or vice versa) by reopening the device and requesting VBI data. For details see Section 1.1.

No replacement exists for `VIDIOCKEY`, and the V4L functions for microcode programming. A new interface for MPEG compression and playback devices is documented in Section 1.9.

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